1. Field of the Invention
The present invention relates to charged particle systems, and more particularly to a stage suitable for use in a charged particle system.
2. Description of Related Art
As the critical dimensions of micro-electronic circuits continue to shrink, accuracy of existing optics-based systems in performing such tasks as identifying defects in patterned substrates (e.g., semiconductor wafers, or optical masks) and measuring critical dimensions (e.g., metal line width, contact hole size) declines. For this reason, charged particle microscopy systems, such as charged particle beam (e.g., electron or ion) systems, with their high imaging resolution have gained popularity.
A charged particle microscopy system typically includes a stage moving in two dimensions (X-Y plane) which aims to fulfill these requirements: (i) high mean time before failure (MTBF), (ii) high speed area coverage, (iii) low mechanical vibrations during travel, (iv) high vacuum (1E-6 torr or less) compatibility, (v) minimal interference (from active or passive, static or alternating sources) with the charged particle microscope (lens and beam), (vi) low particle generation, and (vii) a compact structure.
A high MTBF (MTBF is a measure of durability) of the stage is important considering the large volume of wafers moving through a fabrication facility, the number of process steps at which wafer inspection and CD measurements are required, and the relatively small field of view of the microscope's optics column. These considerations also make high speed area coverage essential in achieving a reasonable throughput. Low mechanical vibration is a prerequisite to accurate measurement by the charged particle microscope where, for example, wafer inspection is carried out while the stage is moving (i.e., the magnitude of the stage vibration must be less than the feature size resolvable by the charged particle microscope). High vacuum compatibility is required because in a charged particle beam system the beam cannot travel through air, thus requiring a vacuum environment. The interference with the charged particle microscope from passive or active, static or alternating sources needs to be minimized to ensure high resolution imaging and precision positioning of the beam. Contamination of wafers due to particles generated by the stage must be kept at a minimum to allow the use of the system in-line in the fabrication facility. Lastly, a stage having a compact structure reduces system foot-print. This is particularly important if the system is to be used in a clean room, since the cost of maintaining a clean room is typically proportional to its size. Also, a smaller stage can be housed in a smaller vacuum chamber which is quicker to pump down to the required vacuum.
The X and Y platforms of a stage are typically driven by either magnetic motors (e.g., linear brushless servo motors) or non-magnetic motors (e.g., piezoelectric motors). Because the fixed and moving components of a brushless magnetic motor do not contact one another during operation, both vibration and particle generation by this type of motor are minimal, and its endurance is increased (no wear and tear). These characteristics along with a magnetic motor's high speed and high torque make it a suitable candidate for driving the X and Y platforms. However, magnetic motors contain strong magnets and have a high permeability housing which can severely interfere with the charged particle microscope optics and beam positioning.
Non-magnetic motors do not contain magnetic materials, and are commonly used for precision stage positioning. However, because the fixed and moving components of non-magnetic motors contact one another during operation, they have lower endurance, more vibration, and more particle generation than magnetic motors. Also, non-magnetic motors are generally slower speed and have less torque than magnetic motors.
FIGS. 1a and 1b show the top (plan) view of a simplified stage 80 which uses magnetic motors to drive both X platform 30 and Y platform 20 (which are stacked). The magnetic motors are located along the edges of the platforms, and are indicated by cross-hatched areas 50a, 50b, 60a, and 60b. In FIG. 1a, the center of a wafer 40 is positioned under a charged particle microscope optics column 10. As indicated by the distances marked as A and B, column 10 is well separated from magnetic motors 50a/b and 60a/b. Therefore, the interference by magnetic motors 50a/b and 60a/b with column 10 is minimal.
However, when the left or right edges of wafer 40 are positioned under column 10 as in FIG. 1b, the distance between column 10 and one of motors 50a/b becomes much shorter (as indicated by the distance marked A). Because of the close proximity of motor 50b with column 10, the magnetic material (e.g., magnet assemblies and magnetic shield of the motor) and electro-magnetic field (e.g., motor coils) of motor 50b interfere with column 10.
The interference from the magnetic motors 50a/b can be reduced by using a wider platform 20 whereby distance A is increased. However, this results in a larger and heavier platform 20 which requires larger motors for driving both platforms 20 and 30. This undermines achieving mechanical precision and the above seven requirements for a stage.
Using non-magnetic motors to drive the X and Y platforms eliminates the interference problem, but causes other inhibiting problems such as mechanical vibration, particle generation, slow area coverage, and low endurance.
Given the shortcomings of each of the magnetic and non-magnetic motors, a stage for use in a charged particle microscopy system is needed which fulfills at least the above-mentioned seven requirements.